Automatic level control



Patented Nov. 24, 1953 AUTOMATIC LEVEL CONTROL Charles W. Harrison, Millington,

to Bell Telephone Laboratories,

N. J., assignor Incorporated,

New York, N. Y., a corporation of New York Application November Z5, 1950, Serial No. 197,608 Claims. (Cl. 179-171) This invention relates to automatic volume or level control systems and, more speciiically, to variable attenuators for use in such systems.

Automatic level control systems are known for regulating the output level of electronic ampliers, whereby undesirably large variations thereof are inhibited. In one common type of automatic level control, there is incorporated in the input circuit of an amplifying stage a variable attenuation network. To achieve control, undesirable variations in the output are utilized to vary the attenuation of this network in a way to maintain a predetermined level. However, it has been characteristic of such networks that changes made to modify the attenuation thereof also effect changes in impedance characteristics. For some applications, it is desirable to utilize a variable attenuation network whose impedance characteristics are not appreciably disturbed by variations in the attenuation level of the network. For example, it may be important in an amplifier circuit to maintain accurate impedance matches in coupling to avoid reflections and other undesirable effects which result from mismatched coupling.

Accordingly, one object of this invention is to provide a variable attenuation network whose impedance characteristics remain constant despite variations in the attenuation level.

A related object is to avoid mismatches in the use of automatic level control systems.

Still another object is to provide an improved automatic level control system.

The variable attenuation network which is the principal feature of the present invention comprises a balanced bridged-T network having two fixed impedance series arms, a variable impedance bridging arm, and a variable mpedance shunt arm. The attenuation level of the network is controlled by varying the impedance of the bridging arm. The input and output impedances characteristic of the network are maintained constant as the bridging arm impedance is changed by varying substantially simultaneously in a predetermined way the impedance of the shunt arm. `For this purpose, there is utilized the fact that these input and output impedances remain constant so long as the network is kept balanced. An unbalance in this network is rebected by a voltage difference across one of the series arms which is normally balanced. Accordingly, in the present invention, any such voltage difference is utilized to vary the impedance of the shunt arm for restoring the balance.

In a preferred species of such a network, thermistors serve as the variable impedance elements network. 'provided a direct-current voutput impedances are For a perfect match, the terminals of the net'VA terminated in their respective which make up the bridging and shunt arms.

In one aspect of the invention, such an attenuation network is utilized in the input stage of an amplifier whose output level is to be controlled.

vVariations from a predetermined output level are utilized to vary the heating current of the bridging arm thermistor in a direction to adjust the attenuation level of the network to a value which provides the desired output level. Moreover, by varying the resistance of the bridging arm thermistor, there is effected an unbalance in the balanced network which produces a voltage dnrerence across one of the xed impedance series arms. This unbalance is detected, amplified, and utilized to control the heating current owing in the shunt arm thermistor in a manner to restore 'thereby maintain substantially un-A b-alance and disturbed the impedance characteristics of the In one embodiment, there is further attenuation network to provide a direct current therein which facilitates detection and utilization of any series arm unbalance.

The invention will be better understood from in connection with forming a part thereof, in which:

Figs. lA and 1B are schematic circuits useful 'Y in describing the principles of the invention;

Fig. 2 shows, in schematic form, a bridged T thermistor type automatic level control circuit in accordance with the invention;

Fig. 3 shows diagrammatically an exemplary embodiment of the invention adapted for radio frequency signals; and

Fig. i shows the equivalent bridge circuit of the impedance balancing arrangement of Fig. 3.

Referring more particularly to the drawing Fig. 1A shows a bridged-T network in which the impcdances Z3 and Z4 form the two series arms, the impedance Z6, in shunt across the series arms, forms the bridging arm, and the impedance Z5 the shunt arm. The effective input and designated Z1 and Z2.

work should be input and output impedances. If a voltage supply Es is inserted in the input of the network,

the condition for balance is that there be no,v

voltage developed across the series arm irrt# pedance Z4. The bridged-T network, for this case, may be represented by the arrangement shown in Fig. 2 which comprises the series impedances Z3 and Z5 in parallel with the series voltage source in the 4 3 impedances Ze and Z2. It can then be shown that the condition for balance is:

It can be seen further that for a match at the input and output terminals. it is necessary'that:

By the use of Equation 1, EquationsIZTand-Gclan be rid of the term Z3, and there farederivedlnew expressions as follows:

transmission 'attenuationisl te bje "val-ies and "zr arid z2 fixed. nente, in 'generar ze earn-tot he made'to satisfy the rrsultant` ex'iiress'ion. 'Howeijr, lifitls i'equ'i'r'edthat zir'zizs, the resultant eriiiression can be 'ihetfa'nd there I is 4also provided an additional v'degree' 'of' freedor'n nece'ssary ,to Aperrnit "impedance matches with variableattenu'ation. v f i "'rlg. 2' shwssherhatieany hjy `way jof "exarhn plea thermistcrtype "automatic 'level 'control circuit inaccordance with one aspect oftlie invention. `The attenuator element comnii's'es the'bridged-T"nettvoikfh ving the two" Series arhis'il 'and "t2, theE insermflainparallel across these serisaimsjand the shunt arm Il. in this exemplary embodiment, 4the "two the bridging arm includesavar'iable vimpedance made up of the' ihdirctly'heated theimistor'Tt, and the shunt 'arm similariy'ineiuees evariable iniiedance 'consistinglof the indirectly heated thrmi'ster T2. to vtheir'rput terminals AA ofthe 'If/network, andmthe'"outputv therefrom :is supplied 'by`way `df thefeutputterminais BB' 'te the input ofthe ar'nlifler'nl, across Whose output terminals CC isheveieped the'voitage et. Inaccordance'with` the'analys's set forth "hereinabove forf'operatiohfthe vtherrhlst'ofs T1 and T2 are chosen `so that" both the input impedance seen at the terminals lAA' 'and the output impedance "seen at the terminals `BB areA equal toi'Zu, the impedance of each of theseries arms IIa'nd I2. )To derive the' advantagesof the inventiorritis deslied 'tdi/ary the attenuation ofthe bridged-1T network ln order vto keep the voltage Vee substantially atl'apr'edetermlned level without Aaf-l.

The input vrsignals are V`supplied 4 fecting the impedance Zt seen at the terminals AA and BB.

As the Voltage eo varies outside its predeter mined limits, this variation is communicated to 5 the heater unit H1, which is connected across the output terminals CC' and which controls the current supplied to the heating element I5 of the thermiston'i. rllhis 'unit can'be any arrangement Which will respond to control volttlnrages to provide heating currents. The charateristics of the heater unit H1 and the ther- "mitrTl eelbhosen so that the resistance of `the thermistor T1 is varied sufficiently to modify 'the attiatinfof the bridged-T network in a 15 vay"tofiestoretheoutput voltage eo to within its predetermined limits. In this Way, automatic leveletntroll J Additionally, in order to realize the advantagls'fA this invention, measures must be taken to maintain substantially undisturbed the in- 25 resistance of the theifmistor T11-act to A'unbal-ance thefbrdged-T network and -set up avolta'geE in the 'series Iarm I2. `-To rutlizethisunbalance, there iseonnected acrossthls Lseries arm lzjtlrefheater vunit Hz'w'hich fcontrolsthe curthermistorfflz. By 5t-his arrangement 'the unbalance voltage Efis usedto -varytl'ie yresistance of thetherrnistor T2. A"lh'e*characterlsticsfof the heater unit Hyand the-ther'rrietori'lzfare chosen vso thatthe "thermistor'fTz 2is*nia-intained.

at a resistance .f Z0 RTV? RTI 40"Where "Rr, "is Athe resistance ofithe *thermistor "match is maintaineddespite the changing attenuationff the network. K

Fig; 3 illustrates 1a circuit arrangement for' autornatic I"level control l-Wliich -has *been ""designefd I particularly for application 'atradio' frequencies.

the'y indirectlylieatedi-'thermlstor T2. AThe Ainput 7'()` decrease A in current fcau'ses f an increase .vin the signals are fsuspnedte the linput:terhumuswel' the resistance RT, of the bridging arm thermistor T1 will result in a mismatch unless the value of the resistance RT, of the shunt arm thermistor T2 is made to adjust itself to the new value of RTl Although the unbalance of signal frequency developed across the series arm I2 can be used to control the thermistor T2 at high frequencies, it may be advantageous to utilize separate means to determine the degree of unbalance of mismatch. By introducing in the attenuator network a second voltage e, at a frequency appreciably lower than that of the signal frequency to be transmitted through the attenuator, the unbalance voltage E can be sampled with greater ease and without detrimental eiects to the transmitted signal. The arrangement in Fig. 3 includes a direct-current source 2U having effectively an internal impedance Zo for providing a second voltage e which will produce a direct-current imbalance voltage E. This unbalance voltage is applied between the cathode and control grid of the tube V3 whose plate current supplies the heating element of the thermistor T2. The inductances Il and IS are used to isolate the tube V3 from the radio frequency unbalance that can exist simultaneously across the series arm I2. As a matter or fact, if the control currents also are fed through choke coils, it would be possible to utilize drect heating type thermistors. In Fig. fl, there is illustrated the equivalent bridge balancing circuit which results from the arrangement of Fig. 3. For the case in which the resistance RT, increases, the voltage E is positive and increases the plate current of tube V3. Ilhis results in a reduction of resistance R112, which in turn will reestablish the balance of the bridge. In this way, automatic level control can be realized without disturbing the impedance match.

It is to be understood that the above-described arrangements are merely illustrative of the principles of the invention. Other arrangements and other applications of such a variable attenuator can be devised by one skilled in the art without departing from the spirit and scope of the invention. In addition, it can be appreciated that a number of such attenuator elements can be used in tandem to increase the range of attenuation.

What is claimed is:

l. An arrangement for attenuation control comprising a balanced bridged-T network having two impedance series arms, a variable 1mpedance bridging arm, and a variable impedance shunt arm, means for varying the impedance of the bridging arm for changing the attenuation of the network and for producing a voltage unbalance across the normally balanced series arm, and means for utilizing said voltage unbalance for varying the impedance of the shunt arm and readjusting the network to balance.

2. An arrangement for attenuation control comprising a balanced bridged-T network having two xed impedance series arms, a var1able impedance bridging arm, and a variable impedance shunt arm, means for applying a voltage across the input of said network, means for varying the impedance of the bridging arm for changing the attenuation of the network and for producing an unbalance across the normally balanced series arm, and means for utilizing said unbalance lfor varying the impedance of the" shunt arm and readjusting the network to balance.

3. A variable attenuator comprising a balanced bridged-T network having two rixed impedance series arms, a variable impedance bridging arm, a Variable impedance shunt arm, means for varying the impedance of the bridging arm for changing the attenuation of the network and producing a voltage unbalance across the normally balanced series arm, and means utilizing said unbalance for varying the impedance of the shunt arm and reacljusting the balance for restoring the impedance characteristics of the network.

e". A variable attenuator comprising a balanced bridged-T network having two xed impedance series arms, a Variable impedance thermister in the bridging arm, and a variable impedance thermistor in the shunt arm, means for applying a voltage to the input of said network, means for varying the impedance of the thermistor bridging arm for changing the attenuation of the network, means for utilizing the unbalance voltage developed across the normally balanced series arm as a result of varying the impedance of the thermistor bridging arm for varying the impedance of the thermistor shunt arm and readjusting the balance of the network.

5. A level control system comprising an amplifier whose output level is to be controlled, a balanced bridged-T network connected in the input circuit of said amplier and having two iixed impedance series arms, a variable impedance bridging arm, and a variable impedance shunt arm, means for applying voltages to the input of the network for producing output voltages from said ampliiier, means for varying the output voltage or said amplier by varying the impedance of the bridging arm, and means utilizing the unbalance voltage developed across the normally balanced series arm resulting from varying the impedance of the bridging arm for varying the impedance of the shunt arm to readjust the balance of the network.

6. A level control system according to claim 5 in which the variable impedance bridging arm and the variable impedance shunt arm include a thermistor.

'7. An automatic level control system comprising an ampliner whose output level is to be controlled, a balanced bridged-T network connected in the input circuit of said amplifier and having two xed impedance series arms, a variable irnpedance bridging arm, and a variable impedance shunt arm, means for applying input voltages to the input terminals of said network for producing output voltages irom said ampliiier, means for varying the impedance of the bridging arm in accordance with variations in the output level of said amplier, and means utilizing the imbalance across the normally balanced series arm, resulting from varying the bridging arm, for varying the impedance of the shunt arm and readjusting the network to balance.

8. An automatic level control system according to claim 7 in which the variable impedance bridging arm and the variable impedance shunt arm include a thermistor.

9. An automatic level control system comprising an amplifier whose output level is to be controlled, a balanced bridged-T network connected to the input circuit of said amplifier, and having input and output xed impedance series; arms, Aa variable lirmedance bridging arm, and a variable impedance shunt arm, means for applying signa-l voltages to the input terminals yof vsaid. network 'for producing output voltages from -said amplifier, means Afor `inserting a control vD..C. voltage in the .input of .said network, means .controlled -by .the output .level of the ampler for varying the impedance of the bridging arm for maintaining said .output level within predetermined zlevels, means .controlled by Vthe D.C. Aunbalance voltage :across the normally balancedcseriesarm for varying `the impedance of .the shunt `.arm .and :readiusting 'the balance of the network.

10. An automatic level control accordingto claim 9 in which the Variable impdan .bridging 4:alim and .the variable impedance lslflunt .arm each includea .thermiston CHARLES W. HARRISON.

References Cited in the le of this patent AlLINIIED STATES PATENTS 

